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Sensors and Instrumentation, Aircraft/Aerospace and Energy Harvesting, Volume 7: Proceedings of the 37th IMAC, A Conference and Exposition on Structural Dynamics, 2019, the seventh volume of eight from the Conference brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of Shock & Vibration, Aircraft/Aerospace, Energy Harvesting & Dynamic Environments Testing including papers on:Alternative Sensing & Acquisition
Active Controls
Aircraft/Aerospace & Aerospace Testing Techniques
Energy Harvesting



Chapter 1. Historical Review of “Building Block Approach” in Validation for Human Space Flight

The evolution of human spaceflight vehicles including launch vehicles continues to propose a perplexing conundrum in the structural dynamics field. Because of the size and weight of these vehicles, it becomes impossible to perform a ground based modal test that replicates all of the loading events of interest (i.e. liftoff, ascent, staging, etc.). As a result, human spaceflight programs have long relied on “building block approaches” to dynamic model updating and validation. Given the wide interpretation and definition of a “building block” approach to dynamic model validation, this paper reviews the state of art techniques used during the Saturn/Apollo and Space Shuttle dynamic test campaigns and contrasts them with the plans for the Space Launch System (SLS). Some of the lessons learned in each program are presented, in terms of how the building block approach was applied in developing models for stakeholders, using and updating analytical models, and use of other test result outside the dynamic tests.

Joel W. Sills, Matthew S. Allen

Chapter 2. Analytically Investigating Impedance-Matching Test Fixtures

When attempting to recreate an operational environment using a test in the lab there can be many problems to overcome. Of particular importance, particularly when testing components, are the boundary conditions, including both the test fixtures and the excitation methods. This paper is an investigation into “N+1” style test fixtures, and the effect of different lengths of fixture under various excitation and control methods. The investigation uses the Box Assembly and Removable Component (BARC) hardware design from the Boundary Conditions Challenge, and is carried out using Finite Element Analysis.

Thomas M. Hall

Chapter 3. Harmonic Forcing of Damped Non-homogeneous Elastic Rods

This work is one of an ongoing series of investigations on the motions of non-homogeneous structures. In the series, natural frequencies, mode shapes and frequency response functions (FRFs) were determined for undamped segmented rods and beams, using analytic and numerical approaches. These structures are composed of stacked cells, which may have distinct geometric and material properties. Here, the steady state response, due to harmonic forcing, of a segmented damped rod is investigated. The objective is the determination of FRFs for the system. Two methods are employed. The first uses the displacement differential equations for each segment, where boundary and interface continuity conditions are used to determine the constants involved in the solutions. Then the response as a function of forcing frequency can be obtained. This procedure is unwieldy and may become unpractical for arbitrary spatial forcing functions. The second approach uses logistic functions to model the segment discontinuities. This leads to a single partial differential equation with variable coefficients, which is solved numerically using MAPLE® software. For free-fixed boundary conditions and spatially constant force good agreement is found between the methods. The continuously varying functions approach is then used to obtain the response for a spatially varying force.

Arnaldo J. Mazzei, Richard A. Scott

Chapter 4. Effects of Multi-Axial versus Single-Axial Excitation of Jointed Systems

In many structural assembly, the main cause of nonlinearities are jointed interfaces. The nonlinearity manifests as a softening response, which is caused by the joint slipping when stick caused by friction forces is overcome. To quantify the friction nonlinearity, the system is typically experimentally tested in each axis independently. This practice assumes that the nonlinearity is directionally uncoupled and can be modeled using the superposition principle. Previous works have shown this is an incorrect assumption. Joints in structural assemblies may not experience excitation from only one direction at a time. This paper experimentally assesses the slip condition of the removable component of the Box Assembly with Removable Component benchmark structure. The slip condition is first studied using single-axis excitation and superposition assumption, and then studies the slip condition under multi-axis excitation. The results show that the slip condition changes when the structure is excited using multi-directional excitation.

S. A. Smith, M. R. W. Brake

Chapter 5. Quantifying the Effect of Component Inertial Properties on System Level Dynamics

Structures are subject to many environments in the lifetime of an assembly, and mechanical environments such as vibration are particularly significant when considering structural integrity. In the early development cycle, mechanical environment test specifications are often derived from assemblies with simplified “mass mock” components. The assumptions for these simplified components generally mimic total mass and center of gravity, but do not always capture moments of inertia. Historically, environments for mass mock components are enveloped and used for future iterations of the true component’s qualification. This work aims to understand and characterize differences in dynamic response due to changes in inertial properties of a component. The FEM of a test structure for this work includes a system level model with true components that will be compared to a FEM with mass mock components. Both versions of the structure will be evaluated based on dynamic response at the component and system levels. The validity and limitations of using mass mock components with approximate inertial properties for deriving environmental specifications will be explored.

Jacquelyn R. Moore, Tyler F. Schoenherr, Darrius Smith-Stamps

Chapter 6. A Method for Determining Impact Force for Single and Tri Axis Resonant Plate Shock Simulations

In the past year, resonant plate tests designed to excite all three axes simultaneously have become increasingly popular at Sandia National Labs. Historically, only one axis was tested at a time, but unintended off axis responses were generated. In order to control the off-axis motion so that off-axis responses were created which satisfy appropriate test specifications, the test setup has to be iteratively modified so that the coupling between axes was desired. The iterative modifications were done with modeling and simulation. To model the resonant plate test, an accurate forcing function must be specified. For resonant plate shock experiments, the input force of the projectile impacting the plate is prohibitively difficult to measure in situ. To improve on current simulation results, a method to use contact forces from an explicit simulation as an input load was implemented. This work covers an overview and background of three axes resonant plate shock tests, their design, their value in experiments, and the difficulties faced in simulating them. The work also covers a summary of contact force implementation in an explicit dynamics code and how it is used to evaluate an input force for a three axes resonant plate simulation. The results from the work show 3D finite element projectile and impact block interactions as well as simulation shock response data compared to experimental shock response data.

Brian A. Ferri, Ronald N. Hopkins

Chapter 7. Non-stationarity and non-Gaussianity in Vibration Fatigue

In vibration fatigue the frequency contents of dynamic loading and structure’s dynamic response overlap, resulting in amplified stress loads of the structure. Time domain fatigue approach does not give a good insight into the underlying mechanics of failure and therefore recently vibration fatigue in frequency domain is getting a lot of scientific attention. Gaussianity and stationarity assumptions are applied in frequency-domain methods for obtaining dynamic structure’s response and frequency-domain methods for calculating damage accumulation rate. However, in application, the structures are excited with non-Gaussian and non-stationary loads and this study addresses the effects of such dynamic excitation to experimental time-to-failure of a structure.The influence of non-Gaussian, but stationary excitation, is experimentally studied via excitation signals with equal power density spectrum and different values of kurtosis. The non-Gaussianity was found not to significantly change the structure’s time-to-failure and therefore, the study focuses on the non-stationary excitation signals that are also inherently non-Gaussian. The non-stationarity of excitation was achieved by amplitude modulation and significantly shorter times-to-failure were observed when compared to experiments with stationary non-Gaussian excitation.Additionally, the structure’s time-to-failure varied with the rate of the amplitude modulation. To oversee this phenomenon the presented study proposes a non-stationarity index which can be obtained from the excitation time history. The non-stationarity index was experimentally confirmed as a reliable estimator for severity of non-stationary excitation. The non-stationarity index is used to determine if the frequency-domain methods can safely be applied for time-to-failure calculation.

Janko Slavič, Martin Česnik, Lorenzo Capponi, Massimiliano Palmieri, Filippo Cianetti, Miha Boltežar

Chapter 8. Use of Topology Optimization to Design Shock and Vibration Test Fixtures

Engineering designers are responsible for designing parts, components, and systems that perform required functions in their intended field environment. To determine if their design will meet its requirements, the engineer must run a qualification test. For shock and vibration environments, the component or unit under test is connected to a shaker table or shock apparatus and is imparted with a load to simulate the mechanical stress from vibration. A difficulty in this approach is when the stresses in the unit under test cannot be generated by a fixed base boundary condition. A fixed base boundary condition is the approximate boundary condition when the unit under test is affixed to a stiff test fixture and shaker table. To aid in correcting for this error, a flexible fixture needs to be designed to account for the stresses that the unit under test will experience in the field. This paper will use topology optimization to design a test fixture that will minimize the difference between the mechanical impedance of the next level of assembly and the test fixture. The optimized fixture will be compared to the rigid fixture with respect to the test’s ability to produce the field stresses.

Tyler F. Schoenherr, Pete Coffin, Brett Clark

Chapter 9. Electromechanical Impedance Method for Applications in Boundary Condition Replication

One of the most challenging parts of modeling structural dynamics is realistically replicating boundary conditions from either a theoretical or experimental perspective. In a finite element model, the mechanical impedance of bolted joints in an assembly can be modeled, as a first step, as an equivalent spring-damper connection. For relatively simple systems, the parameters of such an approximation are updated such that the dynamic characteristics of the model match with the jointed structure. When the assembled structure is in an operational environment, joints are one of the first components of the assembly to change their dynamic characteristics. As a result, identifying a change in their dynamics and further keeping track of the changes is burdensome. Additionally, if a change is detected, it is equally difficult to modify the structure to its previous state without exhaustive testing.To address some of these issues, the present work leverages coupled electro-mechanical impedance-based techniques to monitor the jointed boundary conditions. In this technique, the mechanical impedance of the assembly is indirectly tracked by measuring the electrical impedance of the attached piezoelectric (PZT) system. In the present study, a PZT patch is bonded to the Box Assembly with a Removable Component (BARC) test structure with ten dry bolt connections. First, a baseline electro-mechanical measurement of the ideal assembly is determined and then the torque of the connecting bolts is then slightly altered. As a result, the dynamic properties of the BARC structure along with electrical impedance response of the PZT changes. The feasibility of tracking these changes and determining the modifications necessary to bring the system to its previous dynamic state is the focus of this work.

Timothy A. Devine, V. V. N. Sriram Malladi, Pablo A. Tarazaga

Chapter 10. Latest Design Trends in Modal Accelerometers for Aircraft Ground Vibration Testing

Accelerometers are widely encountered in structural analysis applications such as modal analysis with vibrational or impact input excitation and operational modal analysis. This paper aims to outline design trends and requirements for acceleration sensors in order to insure optimal structural analysis measurement results. Key parameters for a performing modal sensor are: sensitivity, mass, noise level, amplitude and phase frequency response, as well as thermal transient response, thermal sensitivity response, transverse sensitivity (cross axis), base strain and survivability which will be taken into detailed consideration in this paper.Nowadays three IEPE (Integrated Electronic Piezo Electric) sensor designs can be considered: piezo-ceramic shear, piezo-bending beam and piezo-crystal shear mode sensing elements. Unfortunately, none of the sensor technologies available on the market today will allow for the best of all parameters mentioned earlier. Advantages and disadvantages have to be considered in order to make the optimal choice. Even though Variable Capacitive (VC) MEMS sensors can be used in cases of operational modal analysis at ultra-low frequencies, such as Bridge Structural Testing or Monitoring, only IEPE technology will be in this study.Besides the technical properties of an accelerometer, the handling qualities during installation and removal are extremely important for high channel count systems. Installation time, error rate and reliability for more than 10 years during several tests a year are of special interest for the user. Among the considerations made here, easy monitoring and sensitive axis alignment compared to the overall coordinate system will be examined.The German Aerospace Center (DLR) will illustrate the applicability of accelerometers in context of industrial testing such as Ground Vibration Testing (GVT) of aircraft structures or structural and modal testing of wind turbine blades where innovative methods such as allowing one free adjustable degree of freedom around one rotational axis in order to freely orient the sensitive axis.

Yves Govers, Julian Sinske, Thomas Petzsche

Chapter 11. Test-Based Uncertainty Quantification and Propagation Using Hurty/Craig-Bampton Substructure Representations

This work presents a method for uncertainty propagation that is consistent with the “building-block approach” in which components of a system are tested and validated individually instead of an integrated vehicle test and validation being performed. The approach gives a unified methodology for representing and quantifying uncertainty in a Hurty/Craig-Bampton component based on component test results and propagating the uncertainty in component models into system-level predictions. Uncertainty in the Hurty/Craig-Bampton representations is quantified using a new hybrid parametric variation approach based on Soize’s maximum entropy method. The proposed approach combines parametric and nonparametric uncertainty by treating the Hurty/Craig-Bampton fixed-interface eigenvalues as random variables and treating the corresponding mass and stiffness as random matrices. The proposed method offers several advantages over traditional approaches to uncertainty quantification in structural dynamics: the number of parametric random variables is relatively small compared to the usually large number of potential random finite element model parameters; therefore, time-consuming parametric sensitivity studies do not have to be performed. In addition, nonparametric model-form uncertainty is easily included using random matrix theory. The method requires the selection of dispersion values for the Hurty/Craig-Bampton fixed-interface eigenvalues and the corresponding mass and stiffness matrices. Test/analysis frequency error is used to identify the fixed-interface eigenvalue dispersions, and test/analysis cross-orthogonality is used to identify the Hurty/Craig-Bampton stiffness matrix dispersion value. Currently, the mass matrix dispersion is based on engineering judgment, past experience, and historical results. The proposed uncertainty quantification methodology is applied to the Space Launch System liftoff configuration. Robustness of the attitude control system is studied by propagating derived component uncertainty models into gain uncertainty in specific transfer functions relating engine inputs to rate sensors on the core stage, and frequency uncertainty for the fundamental bending and roll modes.

Daniel C. Kammer, Paul Blelloch, Joel W. Sills

Chapter 12. Accumulated Lifetimes in Single-Axis Vibration Testing

Vibration qualification testing verifies and quantifies a system’s longevity in its proposed service environments. Service environments a system could encounter can impart many ranges of excitation in all directions; however, multi-axis excitation testing capabilities for simulating realistic environments are rare and costly. Therefore, multiple, single-axis vibration tests are commonly used to qualify a system and its components to a lifetime of service environments. Quantifying the equivalent amount of time a component has been tested can be difficult when limited to single-axis tests. Further complications arise due to the fact that real-world service conditions are often measured at a system level without instrumentation on each component. In addition, many mechanical systems include joints and contact surfaces that, if altered, can significantly change the component’s vibration characteristics. This makes replicating the boundary conditions of each component difficult. Therefore, another crucial part of single-axis vibration testing is determining boundary conditions to replicate best the real-world environment onto each component. This paper aims to analyze the effects on lifetime estimates using single-axis vibration testing of components under variations in boundary conditions, testing strategies, control locations, and other configuration options. Methods such as power spectral density (PSD), fatigue damage spectrum (FDS), and Miner’s Rule, with quantities such as fatigue cycles, peak response, and RMS response are used to evaluate boundary conditions, study the response of the components, and determine the severity of various test strategies as it pertains to the overall lifetime of the system.

Adam Bouma, Abigail Campbell, Thomas Roberts, Stuart Taylor, Colin Haynes, Dustin Harvey

Chapter 13. Instrumentation and Data Acquisition Mistakes in a Structural Dynamics Facility and How to Learn from Them

Many common mistakes are made as a new engineer or technician using dynamics instrumentation such as accelerometers, pressure transducers, and strain gages. A lot of these are well known or documented and published. Working in a combined-environment testing facility such as AFRL’s Structural Dynamics Lab provides an additional layer of complexity by pushing instrumentation and data acquisition hardware to the limits of their operation parameters. This paper will discuss some of those lessons learned the hard way so that others can learn to not repeat them.

Matthew S. Stefanski

Chapter 14. Adaptive Multi-modal Tuned Mass Dampers Based on Shape Memory Alloys: Design and Validation

The use of shape memory alloys (SMA) is really promising in the field of vibration mitigation. Indeed, several works are already available in the literature, describing how to exploit the special features of SMAs in order to design and build dampers and tuned mass dampers (TMD).Regarding TMDs, the features of SMA materials allow to design adaptive TMDs able to change their eigenfrequencies in order to keep the TMD tuned on the primary system to be damped in case of changes of the dynamic features of the primary system (e.g. changes of the eigenfrequency due to thermal shifts). The possibility to ensure the tuning between the TMD and the primary system allows to achieve an optimal damping action.The adaptive TMDs based on SMAs described in the literature are usually able to work on a single eigenfrequency of the primary system. Conversely, this paper proposes a new adaptive TMD able to change more than one eigenfrequency at the same time with a given level of independence. This allows to work on at least two eigenfrequencies of the primary system, thus realizing a multi-modal adaptive TMD.The paper explains that this multi-modal adaptive TMD is based on a special configuration made from a system of masses and SMA wires. Particularly, each mass is connected to the adjacent masses by SMA wires. The possibility to tune more than one eigenfrequency is achieved by heating/cooling the different SMA wires independently. Indeed, this allows to change the geometry of the adaptive TMD and, at the same time, the tensile load into the SMA wires. This double effect is suitable for building multi-modal adaptive TMDs.The paper first describes the working principle of the adaptive TMD. Then, simulations are presented in order to show the effectiveness of the proposed device.

M. Berardengo, G. E. P. Della Porta, S. Manzoni, M. Vanali

Chapter 15. Application of Transfer Path Analysis Techniques to the Boundary Condition Challenge Problem

A Boundary Condition Challenge Problem was released in May 2017 by Sandia National Laboratories and Kansas City’s National Security Campus (KCNSC). The challenge problem is intended to facilitate collaborative research on methods used for laboratory shock and vibration testing of aerospace components. Specifically, the challenge problem presents a test bed structure consisting of two sub-systems and an applied shock loading. The goal is to replicate the environment observed on one of the sub-systems when it is attached to a different sub-system in a laboratory testing environment.Meanwhile, transfer path analysis (TPA) tools have been available for several decades. TPA techniques are used extensively for noise, vibration and harshness (NVH) engineering in the automotive industry. The techniques provide insight into the vibration transmission of a source excitation to a receiving structure. By re-framing the boundary condition problem into the TPA framework, it becomes clear that TPA tools are directly applicable to the boundary condition challenge problem.

Julie M. Harvie, Maarten van der Seijs

Chapter 16. Testing Summary for the Box Assembly with Removable Component Structure

The boundary conditions of a test will have an effect on the dynamic response of a test unit. The industry standard of designing the most rigid fixture possible may not be the correct approach to replicate responses to service environment loads. The Box Assembly with Removable Component (BARC) structure was developed as a challenge problem for those investigating boundary conditions and their effect on these tests. Several BARC structures have been manufactured and sent to collaborators who have performed a variety of structural tests on the hardware. This paper serves as a collection and comparison of the dynamic testing that has been performed to date by several organizations taking part in this research challenge problem. Of particular interest is the variability in modal parameters between different test articles, as well as any nonlinearities that can be identified.

Daniel P. Rohe, Scott Smith, Matthew R. W. Brake, James DeClerck, Mariano Alvarez Blanco, Tyler F. Schoenherr, Troy J. Skousen

Chapter 17. Comparison of Multi-Axis Testing of the BARC Structure with Varying Boundary Conditions

The Box Assembly with Removable Component (BARC) structure was developed as a challenge problem for those investigating boundary conditions and their effect on structural dynamic tests. To investigate the effects of boundary conditions on the dynamic response of the Removable Component, it was tested in three configurations, each with a different fixture and thus a different boundary condition. A “truth” configuration test with the component attached to its next-level assembly (the Box) was first performed to provide data that multi-axis tests of the component would aim to replicate. The following two tests aimed to reproduce the component responses of the first test through multi-axis testing. The first of these tests is a more “traditional” vibration test with the removable component attached to a “rigid” plate fixture. A second set of these tests replaces the fixture plate with flexible fixtures designed using topology optimization and created using additive manufacturing. These two test approaches are compared back to the truth test to determine how much improvement can be obtained in a laboratory test by using a fixture that is more representative of the compliance of the component’s assembly.

Daniel P. Rohe, Ryan A. Schultz, Tyler F. Schoenherr, Troy J. Skousen, Richard J. Jones

Chapter 18. Strategies for Shaker Placement for Impedance-Matched Multi-Axis Testing

Multi-axis testing is growing in popularity in the testing community due to its ability to better match a complex three-dimensional excitation than a single-axis shaker test. However, with the ability to put a large number of shakers anywhere on the structure, the design space of such a test is enormous. This paper aims to investigate strategies for placement of shakers for a given test using a complex aerospace structure controlled to real environment data. Initially shakers are placed using engineering judgement, and this was found to perform reasonably well. To find shaker setups that improved upon engineering judgement, impact testing was performed at a large number of candidate excitation locations to generate frequency response functions that could be used to perform virtual control studies. In this way, a large number of shaker positions could be evaluated without needing to reposition the shakers each time. A brute force computation of all possible shaker setups was performed to find the set with the lowest error, but the computational cost of this approach is prohibitive for very large candidate shaker sets. Instead, an iterative approach was derived that found a suboptimal set that was nearly as good as the brute force calculation. Finally, an investigation into the number of shakers used for control was performed, which could help determine how many shakers might be necessary to perform a given test.

Daniel P. Rohe, Garrett D. Nelson, Ryan A. Schultz

Chapter 19. Comparison of Vibration Comfort Criteria by Controlled Field Tests on an Existing Long-Span Floor

Problems such as few field tests evidence and irrationality or absence of vibration limits exists in criteria and researches. This experimental study investigated perception thresholds (at the frequency of 5.354 Hz) and the magnitudes of sensation to vibrations caused by an electrical shaker. Linear relationship between vibration magnitude and perception probability has been found. Similar linear relationship exists between mean value of vibration indexes and subject sensation magnitudes. A logarithmic normal distribution of vibration magnitudes causing equal vibration sensation have been found. Limits of criteria are checked, such as ISO 2631/ISO 10137/DIN 4150-2/VDI 2057-1/ATC DG1 et al.

Lei Cao, Jun Chen

Chapter 20. Flight Environments Demonstrator: Part III—Sensitivity of Expansion to Model Accuracy

The ability to extrapolate response data to unmeasured locations has obvious benefits for a range of lab and field experiments. This is typically done using an expansion process utilizing some type of transformation matrix, which typically comes from mode shapes of a finite element model. While methods exist to perform expansion, it is still not commonplace, perhaps due to a lack of experience using expansion tools or a lack of understanding of the sensitivities of the problem setup on results. To assess the applicability of expansion in a variety of real-world test scenarios, it is necessary to determine the level of perturbation or error the finite element model can sustain while maintaining accuracy in the expanded results. To this end, the structure model’s boundary conditions, joint stiffness, and material properties were altered to determine the range of discrepancies allowable before the expanded results differed significantly from the measurements. The effect of improper implementations of the expansion procedure on accuracy is also explored. This study allows for better insights on prospective use cases and possible pitfalls when implementing the expansion procedure.

Debby Fowler, Ryan A. Schultz, Brandon Zwink, Brian C. Owens

Chapter 21. A Demonstration of Force Estimation and Regularization Methods for Multi-Shaker Testing

Design of multiple-input/multiple-output vibration experiments, such as impedance matched multi-axis testing and multi-shaker testing, rely on a force estimation calculation which is typically executed using a direct inverse approach. Force estimation can be performed multiple ways, each method providing some different tradeoff between response accuracy and input forces. Additionally, there are ways to improve the numerics of the problem with regularization techniques which can reduce errors incurred from poor conditioning of the system frequency response matrix. This paper explores several different force estimation methods and compares several regularization approaches using a simple multiple-input/multiple-output dynamic system, demonstrating the effects on the predicted inputs and responses.

Ryan A. Schultz

Chapter 22. Input Signal Synthesis for Open-Loop Multiple-Input/Multiple-Output Testing

Many in the structural dynamics community are currently researching a range of multiple-input/multiple-output problems and largely rely on commercially-available closed-loop controllers to execute their experiments. Generally, these commercially-available control systems are robust and prove adequate for a wide variety of testing. However, with the development of new techniques in this field, researchers will want to exercise these new techniques in laboratory tests. For example, modifying the control or input estimation method can have benefits to the accuracy of control, or provide higher response for a given input. Modification of the control methods is not typically possible in commercially-available control systems, therefore it is desirable to have some methodology available which allows researchers to synthesize input signals for multiple-input/multiple-output experiments. Here, methods for synthesizing multiply-correlated time histories based on desired cross spectral densities are demonstrated and then explored to understand effects of various parameters on the resulting signals, their statistics, and their relation to the specified cross spectral densities. This paper aims to provide researchers with a simple, step-by-step process which can be implemented to generate input signals for open-loop multiple-input/multiple-output experiments.

Ryan A. Schultz, Garrett D. Nelson

Chapter 23. Combining Test and Simulation to Tackle the Challenges Derived from Boundary Conditions Mismatches in Environmental Testing

Recent research stressed out the limitations of current practices on component level environmental vibration testing. These limitations are typically associated with non-realistic excitation mechanisms and the mechanical impedance mismatch due to differences between the operational and the test boundary conditions. General concern is that the real failure modes of the component are not correctly replicated, and more information might be needed to define a representative test practice. Does the current testing practice provide sufficient information? Is there a way to overcome the impedance mismatch between operational conditions and the test configuration by means of simulations and adequate control strategy for environmental tests? This work presents recent results from an intensive test campaign performed on the Box Assembly with Removable Component (BARC). Limitations of state-of-the-art random vibration testing techniques are investigated and Multiple-Input Multiple-Output Random control strategies are combined with simulation tools to find potential research directions to overcome the limitations. The final goal intends to tackle a rationale, rather than a single specific solution, to assess the design of a testing methodology leading to structural responses which are more representative of the operational environment in terms of potential failure mechanisms.

Umberto Musella, Mariano Alvarez Blanco, Davide Mastrodicasa, Giovanni Monco, Di Lorenzo Emilio, Manzato Simone, Bart Peeters, Emiliano Mucchi, Patrick Guillaume

Chapter 24. Defining Component Environments and Margin Through Zemblanic Consideration of Function Spaces

Historically the qualification process for vehicles carrying vulnerable components has centered around the Shock Response Spectrum (SRS) and qualification consisted of devising a collection of tests whose collective SRS enveloped the qualification SRS. This involves selecting whatever tests are convenient that will envelope the qualification SRS over at least part of its spectrum; this selection is without any consideration of the details of structural response or the nature of anticipated failure of its components. It is asserted that this approach often leads to over-testing, however, as has been pointed out several times in the literature, this approach may not even be conservative.Given the advances in computational and experimental technology in the last several decades, it would be appropriate to seek some strategy of test selection that does account for structural response and failure mechanism and that pushes against the vulnerabilities of that specific structure. A strategy for such a zemblanic (zemblanity is the opposite of serendipity, the faculty of making unhappy, unlucky and expected discoveries by design) approach is presented.

Michael J. Starr, Daniel J. Segalman

Chapter 25. European Service Module: Structural Test Article (E-STA) Building Block Test Approach and Model Correlation Observations

The Orion European Service Module—Structural Test Article (E-STA) underwent sine vibration testing in 2016 using the Mechanical Vibration Facility (MVF) multi-axis shaker system at NASA Glenn Research Center’s (GRC) Plum Brook Station (PBS) Space Power Facility (SPF). The main objective was to verify the structural integrity of the European Service Module (ESM) under sine sweep dynamic qualification vibration testing. A secondary objective was to perform a fixed-base modal survey, while E-STA was still mounted to MVF, in order to achieve a test correlate the finite element model (FEM). To facilitate the E-STA system level correlation effort, a building block test approach was implemented. Modal tests were performed on two major subassemblies, the crew module/launch abort structure (CM/LAS) and the crew module adapter (CMA) mass simulators. These subassembly FEMs were individually correlated and then integrated into the E-STA FEM prior to the start of the E-STA sine vibration test. This paper summarizes the modal testing and model correlation efforts of both of these subassemblies and how the building block approach assisted in the overall correlation of the E-STA FEM. This paper will also cover modeling practices that should be avoided, recommended instrumentation positioning on complex structures, and the importance of the FEM geometrically matching CAD in sufficient detail in order to adequately replicate internal load paths. The goal of this paper is to inform the reader of the hard earned lessons learned and pitfalls to avoid when applying a building block test approach.

James P. Winkel, Samantha A. Bittinger, Vicente J. Suárez, James C. Akers

Chapter 26. Control of Plate Vibrations with Artificial Neural Networks and Piezoelectricity

This paper presents a method for active vibration control of smart thin cantilever plates. For model formulation needed for controller design and simulations, finite difference technique is used on the cantilever plate response calculations. Piezoelectric patches are used on the plate, for which a neural network based control algorithm is formed and a neurocontroller is produced to calculate the required voltage to be applied on the actuator patch. The neurocontroller is trained and run with a Kalman Filter for controlling the structural response. The neurocontroller performance is assessed by comparing the controlled and uncontrolled structural responses when the plate is subjected to various excitations. It is shown that the acceleration response of the cantilever plate is suppressed considerably validating the efficacy of the neurocontroller and the success of the proposed methodology.

Onur Avci, Osama Abdeljaber, Serkan Kiranyaz, Daniel Inman

Chapter 27. Comparing Fixed-Base and Shaker Table Model Correlation Methods Using Jim Beam

The key to any dynamic model correlation is an understanding of how the boundary conditions of the test article interact with the test data. Due to budget and schedule constraints, some spacecraft programs opt to correlate the spacecraft dynamic model during the Environmental Qualification Test, conducted on a large shaker table. While this saves cost to the spacecraft program, the base-drive analysis of the dynamic model incorrectly assumes the boundary condition between the shaker and the spacecraft to be completely fixed, except for the prescribed force input.This paper follows-up research published in IMAC 36, “Comparing Free-Free and Shaker Table Model Correlation Methods using Jim Beam.” In that study a free-free impact modal test, a “fixed” base impact modal test on top of the shaker, and a base-drive vibration test were used to assess Finite Element Model (FEM) correlation using different boundary conditions. The NAVCON Jim Beam, a simple and well characterized structure featured in the round robin tests of IMAC 27, was chosen as the test article. Conclusions showed that due to the non-linear compliance of the shaker table, most time would be spent accounting for the boundary condition in the correlation, rather than correlating the test article itself.Previous testing was conducted with the Jim Beam flush mounted to the shaker table, which restricted the motion of the bending and shear modes. To mitigate this constraint, this paper included the use of “donut” force gauges inserted between the shaker table and the Jim Beam. Not only was the direct force input of the vibration test measured, but the gauges acted as spacers which relieved the constraint on mode shapes caused by contact with the shaker table. This constraint was a source of error in the previous modal data.The premise of this follow-on study is the same as before; to compare test data of the same structure with identical instrumentation across different boundary conditions. First, a fixed base modal impact test of the Jim Beam was conducted on a slip table to approximate a modal plate. During this test the Jim Beam was mounted on four disconnected force gauges to simulate the same bolted interface as the shaker table. Second, the Jim Beam was transferred to a large shaker table and a vibration test was conducted. Results of the two tests were compared to investigate the validity of using environmental test data alone to correlate a dynamic model.

James Ristow, Jessica Gray

Chapter 28. Vibration Reduction for Camera Systems Onboard Small Unmanned Aircraft

The performance of camera vibration isolation systems used on a popular small unmanned aircraft system (sUAS) is presented. The use of sUAS or drones for image collection is becoming ever more popular for hobbyists, as well as in commercial and military operations. Many types and methods for vibration isolation and absorption are used to create a more stable environment to acquire images or video. While many systems promise vibration reduction, few studies have been conducted to measure and evaluate their performance. Therefore, this review will provide data obtained by experimentally measuring the vibration levels of the camera and host aircraft. Using this data the transmissibility is determined and the effectiveness assessed. Along with the experimental data, analytic models of the systems will be generated to allow for the integration into future modeling efforts. The analysis utilized a common airframe used in the UAS community along with frequently used camera mounting systems. The data is obtained from aircraft fully powered and airborne in a hovering or level flight configuration. This study will provide sUAS operators the information required for choosing the most effective camera vibration reduction system and/or method for the system of interest.

William H. Semke

Chapter 29. Flight Environments Demonstrator: Part I—Experiment Design and Test Planning

Flight testing provides an opportunity to characterize a system under realistic, combined environments. Unfortunately, the prospect of characterizing flight environments is often accompanied by restrictive instrumentation budgets, thereby limiting the information collected during flight testing. Instrumentation selection is often a result of bargaining to characterize environments at key locations/sub-systems, but may be inadequate to characterize the overall environments or performance of a system. This work seeks to provide an improved method for flight environment characterization through a hybrid experimental-analytical method, modal response extraction, and model expansion. Topics of discussion will include hardware design, assessment of hardware under flight environments, instrumentation planning, and data acquisition challenges. Ground testing and model updating to provide accurate models for expansion will also be presented.

Brian C. Owens, Randall L. Mayes, Moheimin Khan, D. Gregory Tipton, Brandon Zwink

Chapter 30. Replicating Responses: A Virtual Environmental Test of Unknown Boundary Conditions

Environmental testing focuses on producing more appropriate testing procedures in laboratory settings which result in more accurate verification and validation of high-performance and sensitive equipment. However, replicating operational conditions in a testing space is challenging due to the unknown nature of the excitations and boundary conditions the system is subject to in the field. If the boundaries can be formulated in a way such that the response can be replicated with different boundary conditions, this would allow for testing of these structures in laboratory settings of known and applicable boundary conditions.The present work models a finite element beam under unknown boundary conditions and attempts to match its spectral response given a known boundary conditions (i.e., a fixed-fixed boundary condition). Forcing inputs are derived for cases of specified and unspecified cross spectral densities for a subset of the initial response locations. The response of the fixed-fixed beam at all initial response locations is compared to the unknown boundary condition target responses to determine how well the force recreation fits the unknown response. Future work aims to replicate these scenarios in an experimental setting.

Timothy A. Devine, V. V. N. Sriram Malladi, Pablo A. Tarazaga

Chapter 31. An Approach to Component Testing: An Analytical Study

Component testing is a standard process across many industries for multiple decades. A specification is typically derived from empirical data collected during a system-level test where input levels (usually in terms of acceleration) are prescribed at the mounting interface of the component. The component is then typically mounted to a rigid fixture for testing where fixture modes are designed to not interact with the structural modes of the component. An acknowledged shortcoming of a test of this type is that the rigid fixture does not represent the dynamics of the final integrated system. This shortcoming is typically referred to as the “impedance mis-match problem.”This paper introduces a possible approach to mitigate this shortcoming by designing a fixture assuming that knowledge of the system level dynamics are understood. For the purposes of the analytical study presented in this paper, the system level dynamics are understood well enough to characterize the boundary conditions of the component such that the modal parameters of the component are replicated from the system test in the component test. A fixture design is conceptualized based on the Box Assembly with Removable Component (BARC) hardware distributed by Sandia National Laboratories and Kansas City National Security Campus.

Brandon J. Dilworth, Alexandra Karlicek, Louis Thibault

Chapter 32. Issues in Laboratory Simulation of Field Vibration Data: Experimental Results on a Typical Structure

Laboratory tests are required in order to qualify a given test item before it is exposed to its field vibration environment, a process that is frequently referred to as vibration or environmental testing. In a typical laboratory environment, a given test article is mounted on the vibration exciter through a test fixture thus forming a combined structure. The combined system is then driven according to prescribed testing conditions while the test item dynamic response is continuously monitored such that maximum dynamic strain are confined under safe and desired levels. The process of going from the field to the laboratory involves some key steps, that include but are not limited to: (1) knowledge of the dynamic characteristics of the systems involved; (2) design of test fixture in order to properly attach the test item to the vibration exciter table; (3) definition of suitable laboratory inputs that when applied to the test article are capable of predicting or at least simulate its field response. The article aims to discuss some of these important issues, particularly the important role of modal testing principles in obtaining accurate response models for the structures involved. Reasonably accurate and experimentally verified models certainly allow that further questions be addressed in the processing of simulating field vibration data.

Paulo S. Varoto

Chapter 33. Clamping Force Effects on the Performance of Mechanically Attached Piezoelectric Transducers for Impedance-Based NDE

Impedance-based non-destructive evaluation (NDE) constitutes a generalization of structural health monitoring (SHM), where comparisons between known-healthy reference structures and potentially-defective structures are used to identify damage. The quantity considered by impedance-based NDE is the electrical impedance of a piezoelectric element bonded to the part under test, which is linked to the dynamic vibrational response of the part under test through electromechanical coupling. In this work, the piezoelectric element is not bonded directly to the part under test, but rather to a c-shaped clamp, which is then mechanically attached to the part under test. Under this attachment condition, the effect of clamping force on the sensitivity of the impedance-based evaluation is investigated for machined steel blocks with varying levels of damage severity. The highest clamping force tested (600 lb, 2670 N) was the only condition exhibiting increasing damage metric values with increasing damage severity in the parts under test, suggesting that higher clamping force increases sensitivity to damage.

Charles M. Tenney, Mohammad A. Albakri, Christopher B. Williams, Pablo A. Tarazaga

Chapter 34. Data Based Modeling of Aero Engine Vibration Responses

Data based modeling has garnered increased interests in the last decade in vibration response classification, particularly so if the relationships between the response variables and the forcing functions are complex and dependent on multiple factors. Aero engines are one of the most heavily instrumented parts of an aircraft, and the data from various types of instrumentation across these engines are continuously monitored both offline and online for potential anomalies. Measured aero engine responses vary widely in character and amplitude depending on the operating conditions and prevailing environmental conditions. The majority of the vibration assessment is done via monitoring engine vibration levels to fundamental shaft rotational orders. However, focus on shaft orders in isolation may not expose the full picture when a range of other factors are also known to be in effect during operation of a complex machine such as an aero engine. Various complex relationships exist between different parameters such as vibration, temperature, pressure, etc., which are all captured via different instrumentation and vary from engine to engine. A global model to establish the association among the sensitivities of various parameters via their existing instrumentation is highly desirable to establish accurate engine behavior.In the present work, a data-driven approach towards modeling aero engine vibration responses will be presented. This will utilize instrumentation sensor data of various types, which has been generated from a series of test bed runs. Further, empirical relationships and correlation among different types of sensor measurements with the aero engine responses will also be presented.

Manu Krishnan, Ran Jin, Ibrahim A. Sever, Pablo A. Tarazaga

Chapter 35. Experimental Mode Verification (EMV) Using Left-Hand Eigenvectors

Estimation of structural dynamic modes associated with complicated spacecraft and launch vehicle assembly modal test articles is often a challenging endeavor. The mathematical process of modal parameter estimation, applied to measured FRF arrays, yields actual and artificial “noise” modes. Left-hand eigenvectors, estimated as part of the SFD modal identification technique, are used to compute SDOF modal FRFs from (a) the measured FRF array and (b) the estimated “effective” dynamic system. Both graphical displays of candidate SDOF modal FRFs and a modal “coherence” metric provide objective means of separating actual, credible system modes from artificial “noise” modes. Evaluation of modal test data for a Space Launch System component demonstrates the value of the new Experimental Mode Verification (EMV) technique.

Robert N. Coppolino

Chapter 36. Understanding Multi-Axis SRS Testing Results

This paper presents a study done on a round resonant plate fixture used for Shock Response Spectrum (SRS) testing. The goal of this study was to understand the magnitude and character of both on axis and off-axis, with respect to shock input, response of the plate at various locations. The resonant plate was modeled using linear FEA as well as tested experimentally. Tools and approaches based on modal decomposition were developed to understand how the natural frequencies and mode shapes of the structure contribute to the SRS response at a given point and direction on the fixture and/or plate. It is seen that in some instances, the off-axis SRS response can have both a higher amplitude response as well as a different “knee” frequency which can make meeting a designated SRS target very difficult. It is shown that by understanding the modal properties of the plate/fixture assembly, the SRS results can be understood. These results will lead to the capability to predict both the on axis and off-axis SRS response for a given input/output set of locations and eventually the ability to choose the ideal locations to achieve a set of on and off-axis SRS responses to meet a given criteria.

William Larsen, Jason R. Blough, James DeClerck, Charles VanKarsen, David Soine, Richard J. Jones

Chapter 37. Generating Anechoic Traveling Wave in Beams with Various Boundary Conditions

The basilar membrane (BM) is one of the prominent structural members of the inner ear which transports acoustic signals, received by the tympanic membrane in the middle ear, as structural waves to the hair cells. Another characteristic of the BM is its ability to absorb the energy of the structural waves at the apex end of the cochlea. As a result of this biological mechanism, the acoustic energy does not reflect at its boundary and as a result does not resonate back into the BM. This is a key feature of the BM, in particular effects by the helicotrema, that enables humans to hear and comprehend a string of continuous acoustic signals without any overlap. The present work is the result of the inspiration to develop waves propagating in engineering structures with such anechoic characteristics.In a previous study, the authors have established the feasibility of generating such anechoic waves in one-dimensional beams through numerical simulations. This is carried out by augmenting a uniform beam with a spring-damper system. As a continuation to this study, the present work investigates the basis for choosing the spring-damper parameters to absorb structural wave energy. Furthermore, the relationship between various parameters such as location, spring stiffness, and damping coefficient and how they affect the quality of the anechoic waves generated is investigated. The results of this study will lead to a better understanding of anechoic wave generation in finite structures.

Seyedmostafa Motaharibidgoli, V. V. N. Sriram Malladi, Pablo A. Tarazaga

Chapter 38. Flexible and Multipurpose Data Acquisition System Design and Architecture for a Multi-force Testing Facility

Testing facilities that combine simultaneous forces on test articles, such as the Air Force Research Laboratory’s Combined Environment Acoustic Chamber (CEAC), require a data acquisition system that is both robust, flexible, and able to record data at different acquisition rates and store the data separately and simultaneously. This paper will provide an overview of how the Air Force Research Laboratory, Structural Dynamics Lab designed their data acquisition architecture using National Instruments PXIe model hardware and LabVIEW software interface to meet the current and future technical requirements for a flexible combined environment data acquisition system.

Matthew S. Stefanski, Tristan A. Linck

Chapter 39. Monitoring of Environmental and Sound-Induced Vibrations on Artistic Stained Glasses

Stained glasses are a key component of the artistic heritage of most European Christian cathedrals. During the last thousand years they grew in complexity and extension until they reached the size of several square meters. Therefore, artistic stained glasses are one of the elements of cultural heritage that are most exposed to environmental hazard though seldom considered until recent days. One of the modern danger sources for stained glasses are environmental vibrations and sound pressure induced vibrations. Considering the lack of modern literature on this topic, the authors carried out an experimental investigation on the Duomo di Milano stained glasses vibrations. The experimental campaign focused on the dynamic response of glasses due to both environmental vibration and to sound-induced excitation during some events which took place in the big square facing the church. As a result, a preliminary vibration analysis has been computed, thus enabling the characterization of vibrations and their effects under operating conditions. Data show that the response for this type of glass under operating conditions is limited to the 30–200 Hz band, with a concentration of energy in the 40–80 Hz band. Furthermore, considering a 30–200 Hz band, the RMS vibration level due to pop/rock concerts is about 10 times higher than that due to environmental excitation.

Alberto Lavatelli, Emanuele Zappa, Alfredo Cigada, Francesco Canali

Chapter 40. Sensitivity Study of BARC Assembly

This paper will present modal analysis results from a systematic study of the assembly of the Box Assembly with Removable Component (BARC). The paper will present results from testing done with both the cut and un-cut version of the BARC and with the different pieces of the BARC both bolted together and attached with a structural adhesive. The boundary condition will be a fixed base excitation. The results will be presented in terms of both Frequency Response Functions (FRFs) and mode shapes and natural frequencies with a goal of showing how the BARC fixture changes with each assembly modification. Upon completion of this testing it is anticipated that a thorough understanding of how assembly methods change the dynamic response of the fixture. This may lead to a suggested assembly method for anyone testing a BARC fixture.

William Larsen, Jason R. Blough, James DeClerck, Charles VanKarsen, David Soine, Richard J. Jones
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